Lasers are used to drill vias in and ablate material from electronic materials products, such as homogenous films, particulate filled resins, polyimides, and fiber reinforced polymers, either with or without metal cladding. The state of the art for solid state UV laser via drilling, for example, is to set and control or monitor the laser pulse energy by setting and controlling the laser pulse power and repetition rate for a desired process. Gaussian and imaged beam optics configurations are used in this manner. Other laser output performance characteristics vary among different lasers but remain within the specifications of the laser. Variation in laser output performance characteristics within the specified limits can result in differences in drilled via quality among multiple lasers, multiple beam positioning heads, and multiple laser systems, even when the same process is performed and same material is used on all systems. Specifically, variation in parameters within the specifications of the laser can cause, especially for blind via drilling applications, bottom copper cladding residue formation or nonhomogeneous melting, which results in differences in drilled via quality. In these applications, the pulse energy used during the drilling process can also affect via taper (ratio of bottom diameter to top diameter of via) as well as the quality of bottom copper cladding. A high quality blind via is one that has a desired bottom diameter and all thin film resin material removed from the bottom to exhibit a shiny copper cladding spot.
In general, the quality of a laser-processed material refers to a standard of excellence specified by a customer of laser processing equipment. Quality metrics differ for different laser processing operations. In addition to the quality metrics discussed above for via drilling, there are quality metrics for electrically conductive link severing, resistive material trimming, thick or thin film passive component thermal annealing, and substrate material scribing by laser outputs.
The quality metric for link severing governs the extent of operational damage to materials surrounding and underlying a link resulting from laser output interaction with the materials during severing of the link. The quality metric for laser trimming governs the minimization of an electro-optic response contributed by laser interaction during resistive material removal so as not to materially influence electrical measurements performed to determine when a specified resistance value has been achieved during resistive material trimming. The quality metric for laser annealing governs the degree of uniformity of distribution of heat applied to a component to change one of its characteristic parameters. The quality metric for laser scribing governs the minimization of microcrack formation in scribe line trench edges resulting from debris generated during cutting. This is a measure of the difference between laser pulse energy and the ablation energy threshold of the scribed material.
Moreover, the control of pulse energy is imperfect and is subject to short-term transients that may not be normalized by energy control devices present in the optical system. Such deviations in pulse energy can cause undesirable variations in via quality metrics.
What is desired, therefore, are techniques for achieving laser drilling of vias or other removal of material with consistent high quality from multiple lasers, multiple beam positioning heads, and multiple laser-based systems. It is also desired to achieve consistent removal of material by a laser, the output performance characteristics of which vary within specified limits over time.